Why does Blender crash when I append this group?
[I am working on Windows but it was also crashing here.] You have an object called MiddleMirrorPipeHolder that is a child of ParabolicMirrors - but - you also use MiddleMirrorPipeHolder as the mirror object for ParabolicMirrors. I believe that is confusing Blender. Removing the parenting will allow you to append your model. I suggest you use an empty as mirror object.